1. Technical Field
The present invention relates in general to circuit board testing and two-mode actuator control and, in particular, to a method and apparatus to enhance the probing speed of a moving actuator used in the testing of circuit boards.
2. Description of the Related Art
With constant advances in packaging technology, the number of layers and complexity of a printed circuit board of a ceramic substrate increase; therefore, the defect rate of the substrate or circuit boards also grows accordingly. Typically, up to 50 percent of all the faults of a circuit board can be found in the bare-board level. Traditionally, the tester uses the bed-of-nails approach, but this type of tester no longer meets the precision and flexibility requirements for testing, while the density of the circuitry on the boards or substrates increases dramatically.
One solution is to use a tester with movable probes because it is flexible and relatively inexpensive. Typically, this type of tester consists of positioning devices to provide planar motion and an actuator to move the test probe in the vertical direction. In high-speed testing applications, the impact force, steady-state probe force, and rebound of the test probe are considered when improving the quality of the tests. The impact force affects the depth of the footprint caused by the test probe, while the steady-state probe force is required for good electrical contact between the probe and the device surface. Appropriate control of the impact force is necessary for it might cause damage on the device under test (DUT). The steady-state probe force influences the consistency of electrical tests and should be well regulated. The rebound of the test probe determines the tester throughput since the electrical test cannot be performed until the test probe settles.
A typical commercial probing tester, such as Kollmorgan tester system,now offered by Integri-Test, or Teledyne Tac tester, consists of an actuator and a buckling spring probe. The actuator moves between two fixed positions with open-loop control. The impact force, probe contact force, and probe rebound can only be regulated by mechanical linkages and the spring probe. There are two drawbacks of this approach. One, the tester is not flexible since any change, such as the probe contact force, requires redesign of the mechanical components. Two, a small impact force demands a very light probe mass, which is difficult to design. An alternative is to control the probe actuator with an active servo system, which provides programmable probe contact force and regulates the impact force and probe settling.
There are two types of control systems for these probe actuator motions: open-loop and closed-loop. In an open-loop system, the actual position of the probe is ignored and only the applied force is defined. In a closed-loop system, the position of the probe is used to correct the control signal sent to the probe for positioning.
In an open-loop system, because only the force is used, neither the speed at which the probe is actuated nor the motion characteristics of the actuator can be controlled. When high-speed actuation is performed, often the probe will bounce on the surface, both damaging the surface and requiring a worst-case allowance for settling time because the actual position of the probe is not known.
While a closed-loop system allows for an exact positioning of the probe, there can be a problem when the probe encounters a target and the control algorithm tries to position the probe. If the probe is positioned to just contact the site, the force applied to the surface is insufficient to allow for the good electrical contact needed for electrical testing. If the closed-loop system tries to position the probe below the surface in order to have sufficient force to make good electrical contact, the system will tend to build up force and potentially damage the site. Additionally, there will be a tendency of the system to oscillate on the site as it continues to make positional corrections and a high-frequency scraping of the surface will occur.
Systems using probes to test (such as point-to-point electrical tests) require that the probe be retracted from contact with one test site before being relocated to the next test site in order to prevent damage to the device under test. After the positioning of the probe over the test site, it is the motion up and down of the probe which accounts for a significant amount of the time expended in these probing systems. Additionally, it is the motion of bringing the probe into contact with the surface which causes all of the damage to the test site.
In order to increase the speed of the probing system, a method was developed which constrains the environment in a way which maximizes the actuation speed. Specifically, an exact height of the test site and the exact position of the probe was determined. During the probing activity, the probe must be moved away from the device being tested such that relocating the probe will not result in collisions with topographical features of the device. When the constraints are applied, the probe is retracted only to the point necessary to insure that the product is not damaged during relocation of the probe. The probe is then driven back into contact with the device when the XY location of the next test site has been achieved.
When driving the probe into the device, two types of damage occur. The first type of damage is impact damage, or that due to the mass of the probe being stopped by what is being tested. This type of damage is increased as the speed of the probe is increased. The second source of damage results from the force applied to the probe as it is driven by the actuator to maintain contact with the test point (as described in the closed-loop description).
The first problem, impact damage, was initially addressed by using a spring-loaded probe to absorb the shock of the impact of the probe to the surface. This approach provided some improvement, but not enough to provide the increases in speed which were desired. One major liability of this approach was the inability of the system to control the amount of force required by the different types of electrical measurements which were being performed by the system.